Polypropylene-based Thermoplastic Olefin Compositions (TPO) are blends of polypropylene, an elastomer, optional propylene-based elastomers, and optional non-polymeric components such as fillers and other compounding ingredients. Included in the category of TPO compositions are so called impact copolymers (“ICP”) compositions, typically non-cured. Typically, TPOs are multiphase polymer blends where a polypropylene forms a continuous matrix phase and the elastomer component, generally derived from an ethylene containing interpolymer, is the dispersed component. The polypropylene matrix imparts tensile strength and chemical resistance to the TPO, while the ethylene polymer imparts stiffness and impact resistance. Typically, ICPs and TPOs have a dispersed phase which is not, or only modestly, cross-linked.
Traditionally, very low density ethylene-propylene copolymers and ethylene-propylene-diene terpolymers have been used as the modifier component in TPO compositions. The major market for TPOs is in the manufacture of automotive parts, especially bumper fascia. Other applications include automotive interior components, such as door skin, air bag cover, side pillars and the like. These parts are generally made using an injection molding process. Recently, other ethylene-alpha olefin copolymers have been used, especially very low density ethylene-butene, ethylene-hexene and ethylene-octene copolymers which generally have a lower molecular weight expressed in Melt Index units. The density of these latter polymers is generally less than 0.900 g/cm3, indicative of little, if any residual crystallinity in the polymer. Such low crystallinity polymers tend to agglomerate into large, intractable pieces on storage. To increase efficiency and reduce costs, it is necessary to decrease molding times and reduce wall thickness in the molds. To accomplish these goals, manufacturers have turned to high melt flow polypropylenes (Melt Flow Rate greater than 35 g/10 min). These high melt flow rate (MFR) resins are low in molecular weight and consequently difficult to toughen, resulting in products that have low impact strength. Additionally, conventional traditional modifier components do not have a balance of good low temperature toughness in blend with polypropylene while maintaining pellet stability.
There is a need, therefore, for TPO manufacturers to be able to broaden the scope of polymers available to manufacture end use items with a better balance between the performance of the hetero phase composition in its end use, the processability during conversion of the molten compositions into the end use article, a toughness at low temperature while maintaining pellet stability, and the cost of providing those properties.
References of interest include U.S. Pat. Nos. 6,245,856; 6,288,171; 6,232,402; 5,959,030; US 2009/053959; WO 97/20888; US 2015/0025209, EP 0 792 914, and WO 16/057124.